Butane-1,4-diol can be prepared by direct hydrogenation of maleic anhydride.
According to U.S. Pat. Nos. 2,772,291 and 2,772,292, such a process is carried out at high pressures of up to 700 bar, using nickel molybdate or nickel chromate as the catalyst. The yields of butane-1,4-diol are at most 53%. Substantial amounts of tetrahydrofuran and butyrolactone are formed as by-products.
Using Raney cobalt in a discontinuous process, the process of U.S. Pat. No. 2,772,293, yields about 64% of butane-1,4,-diol, and Raney nickel only yields at most 12%. Moreover, Raney nickel and Raney cobalt are unsuitable for industrial use since they become inactivated by the acids formed from the maleic anhydride.
All these processes are operated at high pressures of 700 to 800 bar, which industrially can only be achieved at great expense, especially in view of the low yields of butane-1,4-diol.
German Auslegeschrift No. 2,133,768 (equivalent to British Pat. No. 1,320,839) proposes cobalt-rhenium and cobalt-rhenium-molybdenum compounds as catalysts stable to acids. However, the reaction product contains only 4 to 14 mole percent of the desired butane-1,4-diol.
A single-stage process using maleic anhydride as the starting material is described in German Offenlegungsschrift No. 2,519,817. It uses catalysts containing elements or compounds of elements of sub-groups VII and VIII of the periodic table, which however are not a subject of the present invention.
In German Offenlegungsschrift No. 2,543,673 (equivalent to British Pat. No. 1,454,440) maleic anhydride is reacted, in a first stage, with lower monohydric aliphatic alcohols in the absence of an esterification catalyst to give the maleic acid dialkyl ester. After the ester is isolated, it is hydrogenated, in a second stage, in the presence of a copper chromite catalyst, to give butanediol. According to German Offenlegungsschrift No. 2,553,959 (equivalent to British Pat. No. 1,464,263), esterification catalysts are additionally used in the first stage of the process described above. For the operation of these two processes it is necessary to isolate the maleic acid dialkyl ester free from acid and water under precise conditions, in order to avoid a loss of activity of the catalyst in the hydrogenation stage. It is expressly stated that copper chromite catalysts can rapidly become inactivated if free acids or acid-containing esters are fed to the hydrogenation stage. The monograph by Zymalkowski "Katalytische Hydrierungen" ("Catalytic Hydrogenations"), Stuttgart (1965), page 116, confirms these disadvantages.